1. Field of the Invention
The present invention relates to a window of an aircraft having an electromagnetic shield and a closing member for an opening, and also relates to a gasket seal for use therein.
2. Description of the Related Art
Aircraft have to be able to fly in safety without occurrence of a malfunction, an unforeseen behavior (up-set), or others in High Intensity Radiated Fields (HIRF), which represents an electro-magnetic environment from radio, television, radar, emitters, and other sources during cruising flight or during takeoff or landing. To this end, HIRF protection measures required in (14 CFR) sections 23.1308, 25.1317, 27.1317, and 29.1317 of High-intensity Radiated Fields (HIRF) protection, which stipulate regulations (airworthiness requirements) of Federal Aviation Administration (FAA), have to be taken.
The importance of protection of electric/electronic systems of aircraft has been significantly increasing in recent years for the following reasons:    1) a greater dependence on electric/electronic systems performing functions required for continued safe flight and landing of the aircraft;    2) a decrease in electromagnetic shielding by a composite material of some kind for use in designing aircraft;    3) an increase in susceptibility (sensitivity) to HIRF of electric/electronic systems accompanied by a higher speed of the operating speed of a data bus and a processor, a higher-density of an IC and a card, and a higher degree of sensitivity of electronic equipment;    4) an expansion of use frequency to a high-frequency band of, in particular, 1 GHz and higher;    5) an increase in severity of an HIRF environment with an increase in the number of RF transmitters and electric power; and    6) an adverse affect on part of aircraft when exposed to HIRF environment.
On the other hand, inside an aircraft, due to radio wave and electromagnetic noise (hereinafter simply referred to as electromagnetic noise) emitted from various electronic equipment, such as portable telephones, game machines, notebook-sized personal computers, and PEDs (Personal Electro Devices), such as an active-type RFID (Radio Frequency IDentification) tag attached to air freight cargo, an adverse effect can occur in, for example, communications with a control tower and communications and control of navigation for flight via a predetermined route. Thus, as well known, passengers are asked to refrain from using various electronic equipment inside aircraft.
Since the airframe of an aircraft is generally formed of metal, electromagnetic noise comes and goes from a cabin (a seat space) to a cockpit (a flight deck) and an avionics bay mainly through a cabin window and a cockpit window. Thus, in order to prevent electromagnetic noise that can be a hindrance from entering the cockpit and the avionics bay, a film made of ITO (Indium Tin Oxide), gold, silver, or the like is provided as being inserted to the cabin window made by laminating a plurality of window panels made of acrylic or the like (for example, refer to Japanese Unexamined Patent Application Publication (Translation of PCT Application) No. 2003-523911).
On the other hand, in order to provide the electromagnetic shield, in a cabin window, a conductive film, which transmits light rays, has an electromagnetic shielding performance is held between window panels. Such the conductive film is composed of a film made of ITO, gold, silver, or the like; conductive fibers (Woven Mesh) plated with copper, nickel or the like; a printed mesh formed by printing an ink containing a conductive filler, such as silver on transparent PET (polyethylene terephthalate) or the like; or an expanded metal (mesh made of metal) made of a punched metal plate.
These conductive films to be used for preventing invasion of electromagnetic noise need to be electrically bonded to the airframe. The conductive films are secured onto a retainer mounting frame made of a conductive material, such as aluminum, along the outer perimeter part of the window with an air-tight gasket seal interposed therebetween, by using fastener members, such as clamps, clips or the like made of a conductive material (for example, refer to U.S. Patent Publication No. 2007/0137117 Specification, U.S. Patent Publication No. 2008/0308677 Specification and U.S. Pat. No. 7,913,385 Specification).
The cabin window is mainly formed by using a stretched acrylic material, and the gasket seal that is fitted to the entire perimeter of the window is made of an EPDM rubber (ethylene-propylene-diene rubber) or a silicone rubber for the purpose of providing an air-tight sealing property, so as to maintain the air pressure inside the cabin from a low pressure of the outside of the airframe and prevent outside rain and moisture from invading therein.
In this case, however, since the normal EPDM rubber and silicone rubber are non-conductive materials, they have no electromagnetic shielding effect. Therefore, electromagnetic waves make the gasket seal function as if it were an opening slot (an invading inlet for electric waves), and in the case of a high-frequency band with electric waves whose wavelength is a half (½ wavelength) or less, the electromagnetic waves, as they are, transmit through the gasket seal member, and invade into the airframe without being attenuated.
For this reason, a method has been proposed in which by mixing a conductive filler (or particle) such as metal or the like with the gasket seal material so as to have a conductivity so that the conductive film and the retainer mounting frame are electrically connected to each other.
However, in the case when a material having a resistivity different from that of clamp and clip members is adopted as a conductive filler to be used for the gasket seal material, galvanic corrosion due to bonded dissimilar metals tends to occur under a moistened, humid or salt-water atmosphere environment.
Commercial conductive rubbers have a conductive filler derived from metals such as pure Ag, Ag/Cu, Ag/Al, Ni/Cu, Ni/Al, Ag/C, Ni/C or the like, or graphite-carbon (C), mixed therein. Between this conductive rubber seal and a bonding surface of a clamp, a clip or the like made of aluminum on which the conductive rubber seal is bonded, the bonded state of dissimilar metals is formed in most cases, with the result that galvanic corrosion tends to occur under a moistened, humid or salt-water atmosphere environment to cause corrosion in an aluminum material on the airframe side.
As a countermeasure for alleviating the galvanic corrosion due to the bonded state of dissimilar metals, a method is proposed in which the bonding surface of an aluminum material is plated with a material, for example, nickel (Ni), tin (TIN) or chromate in order to avoid the anodic index or potential difference; however, high costs are required for the pretreatment (polishing) of the plating, masking treatments, plating processes, and the like. Another method may be proposed in which the aluminum material is changed to corrosion resistance steel (CRES); however, for use in aircrafts, this method causes more demerits such as heavy weight.
In addition to these, another countermeasure is proposed in which a conductive corrosion-resistant sealant is applied onto the aluminum side; however, this method causes demerits such as time-consuming reapplying processes onto clamps and clips required upon exchanging them at the time of regular equipment inspections.
In this manner, for the respective countermeasures for alleviating galvanic corrosion, exclusively-used treatment processes are additionally required, resulting in high costs or an increase in weight.
Moreover, for example, in automobiles or various equipments, etc. of other fields, also, prevention of electromagnetic waves from invading from the outside has been required for a closing member that closes a window or an opening, and may lead to the same problems as those described above in these parts.
The present invention is devised in view of these technical problems, and has an object of providing a window of an aircraft, a closing member for an opening, and a gasket seal that can positively prevent electromagnetic noise, without causing galvanic corrosion relative to other parts on the periphery thereof.